Both cats went bad (the new stainless exhaust connected to the cats were cherry red) on the 92 Z28 5.7 we are restoring and now the motor is missing badly. The major tune up we had done has maybe 400 miles on it. We replaced injectors, wires, plugs, etc... Trying to get this thing finished so my son can drive it to school on the 28th of August.. His first car. Thanks for the input!

 

No, they are post everything. The engine was the problem before the cats and is just worse after you noticed it

 

If the exhaust was cherry red before the cat then it was either way too lean or the cats overheated because too rich/heavy misfiring.

GD

 

Clogged cats will cause the car to not accelerate well at all, but generally don't effect the tune up. The Oxygen sensors could mess up a tune if they aren't reading the oxygen count in the exhaust correctly and cause a rich or lean mixture.

 

Old cats will clog up and restrict exhaust flow, causing the engine to run like crap or even (in my experience) cause the engine to die at higher rpm's.

I had clogged cats and when I downshifts and it got to over 3,000 rpms, the engine would die because it couldn't push the exhaust out fast enough.

 

A clogged Cat on my 91 Mustang GT I had turned pipes red near the cats, but once it was switch had no other effect on the car. It ran like crap, had almost no power, and going to an off-road H-Pipe corrected it all. It was not a glowing bright read like a torch had seen hit it, but a very faint glow that I really only saw at night. That was how I determined it was a bad cat.

 

Thank you for your help. The pipes were red from the cats back. The new stainless is now discolered. I was told to change plugs, rotar, and oil since this has happened.

 

When you replaced the injectors what size did you put in?